The invention relates to a device for measuring a fill level of a liquid in a container having an ultrasonic sensor, wherein the ultrasonic sensor is arranged at the bottom end of a damping cup with a measuring tube, wherein the upper region of the measuring tube has at least one inner ventilation hole and a cover assigned to the inner ventilation hole, which cover surrounds at least the upper end region of the measuring tube, and in turn has at least one outer ventilation hole, wherein at least one screen is present in the cover and is in contact with the top of the cover.
Devices for measuring the fill level of a liquid in a container having an ultrasonic sensor such as the one described above are used particularly in motor vehicles to measure the fill level of the engine oil. In order to determine the oil level in the container, ultrasonic waves are emitted from the bottom of the device and reflected at the phase boundary between the oil and the air above it to be received by the ultrasonic sensor again. The fill level in the container can be determined from the travel time of the ultrasonic waves. However, in an engine which is running, particularly in a motor vehicle, it is difficult if not impossible to determine the oil fill level, because the oil is heavily foamed and the sonic waves are reflected by gas bubbles. The values detected therefore differ widely. In order to be able to take a measurement with readable values, an unambiguous boundary surface between the gas and oil media is required. To achieve this, devices called damping cups are attached to the ultrasonic sensors. Inside these damping cups, which cover the measuring section of the ultrasonic sensor, the liquid that is to be measured is calmed, and communicates with the container via a small opening in the damping cup. Only very few gas bubbles get into the damping cup through the small opening between the damping cup and the container. The fill level in the damping cup is equal to the fill level outside the damping cup, but at the same time slows fluctuations that are caused for example by acceleration or cornering. Typically, an antechamber is also assigned to the damping cup and serves to degas the liquid to be measured, particularly foamed oil, thus creating a liquid inside the measuring section of the damping cups that has been degassed and is relatively bubble-free. The damping cups are typically made of plastic, usually from several parts that have been welded, clipped or bonded together.
A species-related device for measuring a fill level of a liquid is known from DE 10 2013 016 164 A1 for example. In that device, a cap is provided in the upper region of a damping cup, which cap has a centrally positioned measuring tube. The cap or cover particularly prevents foam from getting into the measuring tube from the outside, but it must also have a ventilation hole so that the liquid to be measured can rise and fall in the measuring tube. In order to protect the measuring tube from oil foam or liquid that enters through the outer ventilation hole of the cover, a screen is provided which lies over the top of the cover and extends downwards. In this case, it is in the form of a ring.